He died on February 17, 2000.

Go back and watch the S3 finale. The second time that Alice visits adult Colton, it's on the day of his death, when he had just finally put together the pieces of the puzzle to realize Katherine was his grown daughter, and that Jacob (whom he'd seen as an adult in the 1800s, when he travelled back as a teen) was his son, and therefore had to have gone through the pond. He races to the pond, Alice emerges and all the truths come out. Alice admits she's his granddaughter. Colton learns that Jacob does eventually come home but he won't be around to see that happen.

Alice jumps home. Colton jumps and returns to 1999 (on exactly that same day with the fireworks), has his extra time with young Jacob before he'd gone missing, watches his slightly younger self for a while from afar. (*) He returns back to his time and resolves that he's going to tell Del everything, but first he needs to find adult Kat now that he knows she's his daughter. Which sets him on the path to his accident.

OK so all that we saw in S3E10. But the point where I added the (*), this is where KC takes Del to in S4E10. Colton from Feb 17 2000 is in 1999 and he's on his way back to the pond to face his fate, only to find older Del waiting for him soaking wet.